In response to PAR-06-504 "Enabling Technologies for Tissue Engineering and Regenerative Medicine," we present an enabling technology of tracking stem cells with bioconjugated quantum dots (QDs) by non-destructive and non- invasive imaging, both in vivo and in real time. A common challenge in tissue regeneration, similar to developmental biology, is the tracking of cells and tissues that are developing in real time. Our preliminary data demonstrate that bioconjugated QDs are safe to label human mesenchymal stem cells (hMSCs) during proliferation for several passages, as well as differentiation into chondrocytes and osteoblasts. We also show that QD-labeled hMSCs, injected via the tail vein of SCID mice, can be tracked in vivo and in real-time by a whole body imager system. Furthermore, we were able to track multiplexing QD- labeled hMSCs injected subcutaneously in vivo and in real time. Theses findings lead to our overall hypothesis that stem cells delivered systemically home to local defects and participate in defect healing, which has been difficult to study previously due to the lack of in vivo and real time cell tracking modalities. The overall goal of this proposal is to determine the efficacy of QD labeling and tracking of MSCs in vivo and in real time, and to improve our understanding of the relative contribution of different cell populations to the healing of tissue defect. Our long-term goal is to apply QD labeling for cell tracking as a platform technology for the healing of other tissues such as cardiac, neuronal, and pancreatic, as well as other musculoskeletal tissues such as bone-ligament interface, muscle-tendon junction, etc. Public Health Relevance Statement (provided by applicant): This project has the potential to demonstrate a viable method for labeling stem cells and tracking the development of regenerating tissue in vivo and in real time. It also has a potential to contribute to our understanding of the engineered osteochondral interface that is of critical value in bioengineered replacement of arthritic joints.